Glazing on printing press rollers is a frequently-encountered problem in the course of lithographic printing especially with inks employing calcium-based pigments. It is generally acknowledged that deposits on lithographic printing press rollers are primarily calcium phosphate and/or calcium citrate. The calcium ion source is principally from calcium-based pigments in the ink (e.g. lithol rubine) and, to a smaller extent, from hard water. The inks are, of course, oil based whereas the fountain solutions are prepared from available water supplies (which may be quite hard) and contain phosphoric and/or citric acids.
Calcium deposits on the press rollers and blankets must be periodically removed since they will not allow for the carrying of the proper amount of ink. The condition will read out on the paper sheet as light print--neither turning up the ink nor increasing the pressure will correct the problem. In time, glazing will lead to stripping--a condition in which ink rollers refuse to take ink or do not take ink uniformly.
At the present time, the only effective solution for roller glazing is to periodically clean the roller. This is a costly, labor-intensive operation which entails solvents and even scrubbing with pumice for stubborn deposits. Of course, it would be desirable if the calcium deposits could be eliminated in the first instance and it is to this goal to which the present invention is addressed.
Eliminating citric acid and/or phosphoric acid from the fountain solution may eliminate the glazing problem but will lead to other printing problems. Fountain solutions typically contain phosphoric and/or citric acids because of pH requirements as well as the buffering capacity of such acids. Generally, the fountain solution is maintained at a pH level of 3.5 to 5.0. Any lower and the solution becomes too acidic and may affect the drying agent in the ink. With a higher pH, the the fountain solution is unable to keep the plate clean, thus leading to scumming and tinting.
The use of calcium chelating agents such a ethylene diamine tetraacetic acid (EDTA) results in several other problems. The principal problem is that color shifts in calcium-based pigments commonly employed in lithographic inks will occur due to the chelating action of EDTA. Also, while printing using a bimetallic plate having a copper image area, the chelated calcium salts will deposit on the image area which results in a blending of the image. Moreover, EDTA must be ground into the ink since it is insoluble, thus leading to production problems.